Tuesday, May 27, 2014

Book Review: “The Cosmic Cocktail” by Katherine Freese

Katherine Freese’s “Cosmic Cocktail” lays out the current evidence for dark matter and dark energy, and the status of the relevant experiments. The book excels in the chapter about indirect and direct detection of WIMPs, a class of particles that constitutes the presently best motivated and most popular dark matter candidates. “The Cosmic Cocktail” is is Freese’s first popular science book.

Freese is a specialist in the area of astroparticle physics, and she explains the experimental status for WIMP detection clearly, not leaving out the subtleties in the data interpretation. She integrates her own contributions to the field where appropriate; the balance between her own work and that of others is well met throughout the book.

The book also covers dark energy, and while this part is informative and covers the basics, it is nowhere near as detailed as that about dark matter detection. Along the way to the very recent developments, “The Cosmic Cocktail” introduces the reader to the concepts necessary to understand the physics and relevance of the matter composition of the universe. In the first chapters, Freese explains the time-evolution of the universe, structure formation, the evolution of stars, and the essentials of particle physics necessary to understand matter in the early universe. She adds some historical facts, but the scientific history of the field is not the main theme of the book.

Freese follows the advice to first say what you want to tell them, then tell them, then tell them what you just told them. She regularly reminds the reader of what was explained in earlier chapters, and repeats explanations frequently throughout the book. While this makes it easy to follow the explanations, the alert reader might find the presumed inattention somewhat annoying. The measure of electron volts, for example, is explained at least four times. Several sentences are repeated almost verbatim in various places, for example that “eventually galaxies formed… these galaxies then merged to make clusters and superclusters…” (p. 31) “…eventually this merger lead to the formation of galaxies and clusters of galaxies...” (p. 51) or “Because neutrons are slightly heavier than protons, protons are the more stable of the objects...” (p. 70), “neutrons are a tiny bit heavier than protons… Because protons are lighter, they are the more stable of the two particles.” (p. 76), “Inflation is a period of exponential expansion just after the Big Bang”. “inflationary cosmology… an early accelerating period of the history of the Universe” (p. 202), and so on.

The topics covered in the book are timely, but do not all contribute to the theme of the book, the “cosmic cocktail”. Freese narrates for example the relevance and discovery of the Higgs and the construction details of the four LHC detectors, but does only mention the inflaton in one sentence while inflation itself is explained in two sentences (plus two sentences in an endnote). She covers the OPERA anomaly of faster-than-light neutrino (yes, including the joke about the neutrino entering a bar) and in this context mentions that faster-than-light travel implies violations of causality, confusing readers not familiar with Special Relativity. On the other hand, she does not even name the Tully-Fisher relation, and dedicates only half a sentence to baryon acoustic oscillations.

The book contains some factual errors (3 kilometers are not 5 miles (p. 92), the radius of the Sun is not 10,000 kilometers (p. 95), Hawking radiation is not caused by quantum fluctuations of space-time (p.98), the HESS experiment is not in Europe (p. 170), the possible vacua in the string theory landscape do not all have a different cosmological constant (p. 201)). Several explanations are expressed in unfortunate phrases, eg: “[T]he mass of all galaxies, including our own Milky Way, must be made of dark matter.” (p. 20) All its mass? “Imagine drawing a circle around the [gravitational lens]; the light could pass through any point on that circle.” (p. 22). Circle in which plane?

The metaphors and analogies used by Freese’s are common in the popular science literature: The universe is an expanding balloon or a raisin bread, the Higgs field is “a crowded room of dancing people” or some kind of molasses (p.116). Some explanations are vague “The multiverse perspective is strengthened by theories of inflationary cosmology” (which?) others are misleading, eg, the reader may be left with the idea that Casimir energy causes cosmic acceleration (p. 196) or that “Only with a flat geometry can the universe grow old enough to create the conditions for life to exist.” (p. 44). One has to be very careful (and check the endnote) to extract that she means the spatial geometry has to be almost flat. Redshift at the black hole horizon is often illustrated with somebody sending light signals while falling through the horizon. Freese instead uses sound waves, which adds confusion because sounds needs a medium to travel.

These are minor shortcomings, but they do limit the target group that will benefit from the book. The reader who brings no background knowledge in cosmology and particle physics I am afraid will inevitably stumble at various places.

Freese’s writing style is very individual and breaks with the smooth – some may find too smooth – style that has come to dominate the popular science literature. It takes some getting used to her occasionally quite abrupt changes of narrative direction in the first chapters, but the later chapters are more fluently written. Freese interweaves anecdotes from her personal life with the scientific explanations. Some anecdotes document academic life, others seem to serve no particular purpose other than breaking up the text. The book comes with a light dose of humor that shows mostly in the figures, which contain a skull to illustrate the ‘Death of MACHO’s’, a penguin, and a blurry photo of a potted plant.

The topic of dark energy and dark matter has of course been covered in many books, one may mention Dan Hooper’s “Dark Cosmos” (Smithsonian Books, 2006) and Evalyn Gates “Einstein’s Telescope” (WW Norton, 2009). These two books are meanwhile somewhat out-of-date because the field has developed so quickly, making Freese’s book a relevant update. Both Gates’ and Hooper’s book are more easily accessible and have a smoother narrative than “The Cosmic Cocktail”. Freese demands more of the reader but also gets across more scientific facts.

I counted more than a dozen instances of the word “exciting” throughout the book. I agree that these are indeed exciting times for cosmology and astroparticle physics. Freese’s book is a valuable, non-technical and yet up-to-date review, especially on the topic of dark matter detection.

[Disclaimer: Free review copy. Page numbers in the final version might slightly differ.]

Minor gripe: the above describes a popular (i.e. many people like it) book about science without saying anything about its level of difficulty. (Newton's Principia is a popular science book.) Freese's book, on the other hand, is a popular-science book, i.e. a book about popular science (i.e. science described for a non-expert readership).

Another minor gripe: The age of the universe is a function of lambda, Omega and H. Without further assumptions, it is not clear that it has to be almost flat in order to be so old. (Otherwise, we would have known it must be almost flat before having measured Omega and lambda accurately enough to claim this.)

If the universe were to present an observation would it accept the Nobel Prize for the measure or the spirit of it? Would it smile that at least it was honored as a source of inspiration?

Can we compare flat and round if ultimately they describe the same higher concepts of space on some say dimensional level? Newton did write a popular book in terms of old geometry and not the new calculus but even the calculus can turn square maps into round ones over some concept of time.

In my (admittedly somewhat sketchy) understanding, the value of the CC is one one of the criteria that distinguish the different compactifications, they may have the same CC but still differ in particle content or symmetry groups, correct me if I'm wrong. Some time back there was some talk about the 'string vacuum project' if you recall (and whatever happened to that?) and they'd sample a selection from the 10^500 to see which had a good CC or the right chiral symmetry, etc, but for all I know the CC doesn't necessarily have to be different for all of these. Best,

Well, You should embed the mechanism (or a version of it) in a larger context and a compactification scheme e.g. F-theory flux vacua but that doesn't change the fact that each string vacuum will have a different CC.

Giotis: Do you realize that you are saying string theory predicts that the observed value of the CC can only have been produced with the particle content of the standard model? Saying that one should 'embed' the mechanism into the compactification scheme somehow just means to me that there are different ways to do this embedding, consequently different theories with the same CC.

In general what you are doing is first to construct a model that reproduces roughly the SM gauge group and then try to fix the moduli via fluxes. But these processes are not independent; there is a very complicated interplay between the two and one affects the other in a non-trivial way (additional consistence conditions comes into play too). That’s why people often make simplifications for one process when they are focusing on the other and vise versa. To have the complete setup is a very complicated thing.

In any case the expression that ‘each vacuum in the string theory landscape has a different cosmological constant’ is legitimate; I wouldn't say that the author made a mistake in any reasonable sense.

The question of if an imagined CC varies or not, even if a matter of thought, even if distinct grounds are not resolvable as a unified landscape driving life close to frontiers but not beyond chaos (those busy not being born are dying) is the essential one in our time facing physics in all its greater span and depths of complexity. (as we can cite in the recent literature). But the existence of BICEPS data or not as a reduced independent oriented plane of symmetry or intrinsic chiral spin is the same question if a complete description in all its simplicity.

Apologies for coming in a little late, but I was just wondering about the part of the article that states that quantum fluctuations do not cause Hawking Radiation. I have read in several places that one interpretation of Hawking radiation is virtual particles popping up near the event horizon, with one going in and the other escaping before they can annihilate and return the "borrowed" energy from the vaccum or whatnot. I've also seem an explanation from tunneling, where particles can escape because part of their wavefunction crosses the horizon (or something to that effect).

I'm not sure if one is actually true, both are true and contribute to different degrees, or both are entirely off base. Your comment helped ease worries I've had with the fluctuation idea (such as how the in-falling particle, which I assume still has positive energy even if it is an antiparticle, could somehow contribute negatively to the black holes mass), but I'm still wondering what exactly goes on and why so many popular science sources could get things so spectacularly wrong.

Although Bee can explain better of course, allow me to say that I think she is talking about quantum fluctuations of space time (i.e. of the gravitational field i.e. of the metric) and not of quantum fluctuations in general. What Bee means is that Hawking radiation is not a QG effect. To derive the Hawking radiation you treat space-time as a fixed classical background; you don’t quantize it. You quantize instead the quantum fields living on the fixed classical space-time of a Black hole near its Horizon. The Horizon (the causal structure basically) is what is important for the radiation to be produced and this is a property of the metric of the classical Black hole.

So these various heuristic interpretations of the Hawking radiation you've read in popular books are legitimate; meaning that the Hawking radiation can be attributed to the vacuum quantum fluctuations of the fields near the Horizon.

Of course strictly speaking, if you want, you can quantize even the space time perturbatively (i.e. quantize small fluctuations of the gravitational field around the fixed classical space time of a black hole); then the gravitons as quanta of this field will contribute to the Hawking radiation as any other field.

So the author of the book could save herself embarrassment if she would have said:

“Hawking radiation is caused by quantum fluctuations of space-time as well” :-)

Seriously though, it is very strange that a professional physicist made such a huge mistake. I think it must be a misprint of some sort.

Giotis explanation is correct and is exactly what I meant. Freese writes explicitly "quantum fluctuations of spacetime... cause pairs of particles and antiparticles to pop in and out of existence". The problem is the insertion "of spacetime". Quantum fluctuations of spacetime would be treated in quantum gravity, yet particle-antiparticle production has in general nothing to do with that. Particle and anti-particle pairs are created by quantum fluctuations, period. I am picking at this sentence because it supports the common misunderstanding that Hawking radiation is a quantum gravity effect. It is not. It's quantum field theory in a classical spacetime background. Best,

Thanks, I appreciate the encouragement. If I were to write a book though, it would not be about quantum gravity. It's not that I don't want to, I can't see how to make the time to do it well, and if I don't have the time to do it well, I don't see why do it at all. Best,

Thanks, I see what you mean about the dash, will be more careful with these. About the age of the universe, the book is generally very sketchy on how these parameters are to be understood and how they are extracted from the data. Be that as it may "almost flat" is a vague expression, but mainly I stumbled over the sentence saying that it has to be flat. (And let me not get started on the problem that she doesn't explicitly distinguish between spatial curvature and space-time curvature which I know (mostly from writing this blog) tends to confuse people who come across this first time.) Best,

Thank you very much for the responses. Definitely helps clear things up!

One final question, and this is in relation to the actual mechanism of reducing the black holes mass. I understand the idea of pair creation and capture by a black hole making the particles "real," but if they are both particles with positive energy (change in charge being the only difference), how does the outgoing particle "steal" mass from the black hole. I've yet to ever hear an intuitive explanation for this (I mean what the heck would negative energy for a particle even mean?).

The energy of the infalling particle is negative, roughly, because inside the black hole horizon time and space are interchanged. I don't think there really is a good 'intuitive' explanation for it. I mean, why do you expect to have an intuition about black hole physics? Best,

This is just the matter of coordinate definition. At pictures of the Schwarzschild metric the space-time gets curved monotonously toward singularity as nothing would ever happen with time dimension. At least the gravitational potential is borrowed from global time coordinate at all derivations instead from observer local time.

No, it is not a matter of coordinates. You just continue to demonstrate your lack of understanding. Do yourself a favor and read at least this. It's a property of the space-time, not of the coordinates you chose.